So These Stars Orbit in a Bar...

Astronomers identify the stellar patrons of the Milky Way bar

Forget the restaurant at the end of the Universe — astronomers now have the clearest
understanding yet of the bar at the center of the Milky Way.

Scientists with the Sloan Digital Sky Survey III (SDSS-III) have announced the discovery
of hundreds of stars rapidly moving together in long, looping orbits around the center of
our Galaxy. "The best explanation for their orbits is that these stars are part of the
Milky Way bar," says David Nidever, a Dean B. McLaughin Fellow in the Astronomy Department
at the University of Michigan. "We know that the bar plays an important role in
determining the structure of the Galaxy, so learning more about these stars will help us
understand the whole Galaxy, even out here in the spiral arms."

A map of the innermost Milky Way, with circles
marking the regions explored by the SDSS-III APOGEE project. Circles marked
with "X" show places where the project found high-speed stars associated with
the Milky Way's bar moving away from Earth. The lighter regions marked with
dots on the other side of the Galactic Center show places where the
fourth-generation Sloan Digital Sky Survey hopes to find counterpart bar
stars moving toward the Earth.

Illustration Credit: David Nidever (University of
Michigan / University of Virginia) and the SDSS-III Collaboration.
Background image from the
Two-Micron All Sky Survey Image Mosaic (Infrared Processing and
Analysis Center/Caltech & University of Massachusetts).

The team's discovery came from accurately measuring the speeds of thousands of stars
near the center of the Milky Way. The center of our Galaxy is 30,000 light-years away — close
by cosmic standards — yet we know surprisingly little about it, because the Galaxy's
dusty disk hides it from view. In spite of this blind spot, though, we do know a key fact
about our Galaxy: like many spiral galaxies, the Milky Way has a 'bar' of stars that orbit
together around the Galactic Center.

"We know of the bar's existence from many separate lines of evidence," says Gail Zasowski,
a National Science Foundation postdoctoral Fellow at The Ohio State University. "What we
don't know is which stars are part of the bar, and what the velocities of those stars are.
That information will help us understand how the bar formed, and how its stars relate to
the stars in the rest of the Galaxy."

The trouble is that there is no obvious way to tell a star in the Milky Way's bar apart
from any other star in the same neighborhood. Instead, the key to finding bar stars is to
measure the velocities of many stars, then see whether some of those stars are moving
together in some unusual pattern. Although interstellar dust blocks nearly all visible
light, longer infrared wavelengths can partially shine through. So a survey of stellar
positions and velocities that operates in infrared light could finally pierce the veil of
dust, and collect data from enough stars in the innermost Milky Way to firmly identify
which ones are part of the bar.

Enter SDSS-III's new Apache Point Galactic Evolution Experiment (APOGEE). APOGEE uses a
custom-built high-resolution infrared spectrograph attached to the 2.5-meter Sloan
Foundation Telescope in New Mexico, and is capable of measuring the velocities and
chemical compositions of up to 300 stars at once. "What separates APOGEE from previous
spectroscopic surveys is that we are studying the Galaxy using infrared light," Nidever
says. APOGEE began observations in June 2011 and has already observed more than 48,000
stars all over our galaxy.

In a paper published recently in the Astrophysical Journal, a worldwide team of
scientists including Nidever and Zasowski used data from the first few months of APOGEE
observations to measure the velocities for nearly 5,000 stars near the Galactic center.
With these velocity measurements, they assembled a picture of how these stars orbit the
center of the Milky Way. However, quite unexpectedly, they found that a substantial
fraction of stars in the inner Galaxy are moving away from us quickly — about 10 percent
of the total stars in their sample are moving at more than 200 kilometers per second
(400,000 miles per hour) away from the Earth. The observed pattern of these fast stars is
similar in many different parts of the inner Galaxy, and is the same above and below the
midplane of the Galaxy — suggesting that these measurements of fast central stars are not
just a statistical fluke, but really are a feature of our Galaxy.

An artist's impression of what the Milky Way might look
like viewed from above. The small blue dot is where we are on Earth (not to scale).
The solid red arrows show the high-speed stars moving away from Earth that were
discovered by SDSS-III. The dashed arrows show the stars moving toward
Earth that are expected to be seen by the fourth-generation Sloan Digital
Sky Survey.

Credit: Jordan Raddick (Johns Hopkins University)
and Gail Zasowski (The Ohio State University / University of Virginia).
Milky Way artist's concept by NASA/JPL-Caltech/R. Hurt (SSC-Caltech).

The team then compared their observations with the predictions of the bar stars from the
latest computer models of the Galaxy — and the observations matched the predictions
closely. "Based on the evidence from the model comparisons, I am now confident that these
fast-moving stars are part of the bar," Nidever says. "I was actually quite surprised that
they showed up so clearly in our survey.

APOGEE's identification of which stars are part of the bar will allow astronomers to study
how stars in the bar and in the rest of the galaxy react to one another. "The bar acts
like a giant mixer for our galaxy," says Steven Majewski, a professor of astronomy at the
University of Virginia and the principal investigator for the APOGEE project. "As the bar
rotates, it churns up the motions of nearby stars. Over time, this mixing should have a
large effect on the disk of our galaxy, including in spiral arms where we live, but this
effect is not well understood. This new sample of definitively-identified bar stars gives
us a unique opportunity to learn more about exactly how this giant blender mixes up our
galaxy."

But the team's discovery only tells half the story. So far, APOGEE has only observed one
side of the bar, the side where the stars are moving away from the Earth. On the other
side, the stars must be moving toward Earth. But unfortunately, the Sloan telescope is
inconveniently placed: the other half of the Milky Way bar is visible only from Earth's
southern hemisphere. Seeing the other side of the bar is one of the motivations for a
planned fourth generation of the Sloan Digital Sky Survey. Part of this successor project
will implement the same techniques using a 2.5-meter telescope in Chile to observe the
rest of the inner Milky Way. The new survey is set to begin in 2014.

About SDSS-III

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the
Participating Institutions, the National Science Foundation, and the U.S.
Department of Energy Office of Science. The SDSS-III web site is
http://www.sdss3.org/.

SDSS-III is managed by the Astrophysical Research Consortium for the Participating
Institutions of the SDSS-III Collaboration including the University of Arizona,
the Brazilian Participation Group, Brookhaven National Laboratory, University of
Cambridge, Carnegie Mellon University, University of Florida, the French
Participation Group, the German Participation Group, Harvard University, the
Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA
Participation Group, Johns Hopkins University, Lawrence Berkeley National
Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for
Extraterrestrial Physics, New Mexico State University, New York University, Ohio
State University, Pennsylvania State University, University of Portsmouth,
Princeton University, the Spanish Participation Group, University of Tokyo,
University of Utah, Vanderbilt University, University of Virginia, University of
Washington, and Yale University.

Contacts:

David Nidever,
University of Michigan,
dnidever -at- umich.edu, 734-615-6141